Add __fish_parent_directories helper function for completions

Can be used to retrieve a list of parent paths, useful for searching
ancestors recursively via their absolute paths. Paths are returned from
deepest to shallowest, starting from the path passed in. Paths are not
validated for performance reasons. (Usually the input to
__fish_parent_directories would be (pwd) or (dir $file).)

share/functions/__fish_parent_directories.fish

@@ -0,0 +1,25 @@
+# Generates a list of parent directories for a given path
+# i.e. /a/b/c/d -> [/a/b/c/, /a/b/, /a/, and /a]
+function __fish_parent_directories
+	if test (count $argv) -ne 1
+		# for use in completions, so don't spew error messages
+		return 1
+	end
+
+	set -l splits (string split '/' $argv[1])
+	set -l parents
+
+	for split in $splits
+		if test (string length "$split") -eq 0
+			continue
+		end
+
+		set parents "$parents[1]/$split" $parents
+	end
+
+	for parent in $parents
+		echo $parent
+	end
+
+	return 0
+end